There are many situations in which it is desired to prevent the unintended reception and monitoring of electronic communication. For instance, the value or relevance of certain business, personal, or national defense-related data may be diminished if not maintained in confidence. Various schemes have been developed to reduce the likelihood of unintended reception of such data. One such scheme is known as frequency hopping, an example of which is shown in FIG. 9. According to the frequency hopping algorithm shown therein at reference number 100, a message is divided into a plurality of portions 102 that are transmitted according to a predetermined sequence of frequencies. Each portion 102 is transmitted for a predetermined duration, which is known as a dwell time 104. Unless a receiver knows the predetermined frequency sequence, it may be difficult to obtain enough of the message portions to reconstruct the entire message.
Notwithstanding this difficulty, certain strategies have been developed to identify the frequencies upon which the message is being transmitted. For example, when using the commonly-employed Minimum Shift Keying (MSK) or Gaussian Minimum Shift Keying (GMSK) communication techniques, useful information may be obtained by putting the received signal through a non-linear operation such as a squaring or cubing operation. FIG. 10 depicts a power spectral density (PSD) plot showing the power spectral density of a squared GMSK signal as a function of the ratio of frequency to data rate. Strong harmonics 105, 106, 107 can be seen at half the data rate, 1.5 times the data rate, and 2.5 times the data rate, respectively. An intercept receiver may be programmed to detect and identify strong harmonics, and the original signal may be derived therefrom.
A further level of protection may be realized by varying the transmit frequency while the frequency hopping algorithm dwells on a frequency. This frequency variance is known as “jitter,” and the frequency deviation of the jitter is typically much smaller than the magnitude of frequency change imparted by the frequency hopping algorithm. While many jitter algorithms exist, a preferred algorithm is depicted in FIG. 11 at reference number 110, in which the frequency of each portion 112 gradually increases through a nominal frequency fn throughout the respective dwell time 114. This type of frequency increase is known as a frequency “chirp” because of its resemblance to the increasing frequency of the sound of a bird's chirp. The uniform and compact frequency distribution of the frequency chirp technique, shown in the histogram of FIG. 12, makes the frequency chirp technique preferable over other types of jitter algorithms.
The frequency chirp technique also reduces the detectability of second order harmonics when used with a GMSK signal. FIG. 13 is a PSD diagram of a squared GMSK signal using the frequency chirp technique. The various output lines 116, 118, 120 represent different windowing techniques (such as Blackman Harris, Boxcar, Hanning, and Bartlett) available to an intercept receiver to analyze the phase shift of an incoming signal. Note that the narrowest harmonic, at 115, illustrates the original harmonic content before the frequency chirp technique was applied. It can be seen that using the frequency chirp technique somewhat reduces the detectable magnitude of the second-order harmonics, and arbitrarily good harmonic suppression can be realized depending on the magnitude of the frequency deviation chosen by the chirp. Indeed, the frequency chirp technique may be advantageously used purposes of harmonic suppression. Unfortunately, it is possible for an intercept receiver to be configured to compensate for a frequency chirp, and merely remove the frequency variations of the chirp prior to processing the signal.
It is therefore an object of the invention to reduce the detectability of an electronically transmitted signal.
It is another object of the invention to increase the security of a GMSK signal transmission by reducing the detectability of one or more second-order harmonics.
It is a further object of the invention to reduce the detectability of a signal while maintaining an acceptable level of signal strength.
A feature of the invention is the division of a frequency chirp into a plurality of scrambled transmission periods, where the frequency during each transmission period is either increasing or decreasing.
An advantage of the invention is a reduced ability of an unauthorized receiver to detect a transmitted signal.